Quality of radio reception, particularly in the frequency modulated (FM) field, and especially stereo FM reception in automobiles is decreased by multi-path reception of signals, besides reception of stray signals, such as interfering or noise signals emanating from the ignition systems of vehicles operating close to the vehicle in which a listener wishes to receive a radio program with as little distortion and noise as possible.
Moving vehicles, such as automobiles, have constantly varying conditions of radio reception. Directional antennas, thus, cannot be used. In stationary sets, substantial improvement in quality of reception can be obtained by directional antennas which, however, unfortunately cannot be readily applied to vehicles. Rather, to permit reception regardless of the orientation of the vehicle with respect to a transmitter, antennas for vehicles are so designed that they are essentially immune to the relative orientation of the vehicle with respect to the transmitting antenna. It has been proposed to improve reception by utilizing a plurality of receiving antennas, and feed the signals from the respective antenna to a receiver, which then will be of the type known as a diversity receiver.
Diversity receivers of the prior art select the respective antenna signals solely based on field strength. The selection of the specific antenna from which this signal is obtained thus will not necessarily be that one in which the signal-to-noise ratio is the highest.
It has been previously been proposed--see German Patent Disclosure Document No. DE-OS 35 10 580 --to provide weighting coefficients, and apply the weighted coefficients to phase corrected intermediate frequency (IF) signals derived from two antennas. The signals are then combined or superposed over each other. The weighting coefficients can be controlled or adjusted. The determination of the respective weighting coefficient is made by digital control. A controller checks if small changes in the phase position of the IF signal with respect to each other results in an increase or a decrease of the overall amplitude of the IF signal. If the change results in a decrease of the overall amplitude of the IF signal, the change in phase position is reversed and a change in the opposite direction is initiated. If the change of the phase increases the amplitude, the change is continued in the same direction. Digital control of this type requires a substantial time period since the overall IF signal amplitude must also be checked. This time period becomes excessively long if the initial change of phase was in the wrong direction, that is, resulted in a decrease in the IF signal amplitude.